A finger joint mechanism composed of a motor and a gear drive is used in a robot hand or the like. The torque capacity of the finger joint mechanism is limited by the gear drive that drives a joint in the finger joint mechanism. For this reason, a higher model number must be used for the gear drive in order to increase the torque capacity of the finger joint.
However, using a higher model number for the gear drive increases the outside-diameter dimensions of the joint mechanism because of the increased dimensions of the gear drive. Accordingly, there is a problem in that the torque capacity cannot be increased in the case of a finger joint mechanism that has a small diameter.